Recently, a model of a cell phone for handling a plurality of communication modes such as a WCDMA (Wideband Code Division Multiple Access (trademark)) mode or a GSM (Global System for Mobile Communication (trademark)) mode has been widely used (for example, see Patent Document 1).
The output states of a signal during communication are different in a WCDMA communication mode and a GSM communication mode. The signal output state of the WCDMA communication system is in a mode where power is consumed constantly, whereas a bursty signal output is repeatedly several times within one frame in a GSM mode. Accordingly, since power supply current also flows intermittently or rapidly in response to the above transmission timing, fluctuation in power supply voltage is caused by the internal resistance of a power supply and the internal resistance of a power supply line.
It is ordinary that a communication device such as a mobile telephone also includes other circuits for driving a microphone and a speaker, a display and LEDs, or the like. When fluctuation in power supply voltage is generated as described above, the fluctuation affects the operation of other circuits. To avoid such circumstances, the configurations for supplying power to a communication circuit through a power supply circuit such as a DC/DC converter have been utilized. However, it is necessary to increase in capacity of a power supply circuit for sufficiently suppressing voltage fluctuation along with the supply of large current, and this leads to cost increase.
It is supposed to use a burst current suppression circuit including a RC-circuit, as shown in FIG. 9, for suppressing fluctuation in power supply voltage without increasing in capacity of a power supply circuit. A power supply (i.e., a battery) 1 supplies power to a communication device 3 and other circuits 4 through a power supply circuit 2; however, the power supply 1 supplies power to the communication device 3 through a burst current suppression circuit 5. By the operation of the burst current suppression circuit 5, the current consumed by the communication device 3 is supplied by an electric charge to be charged in a capacitor 5C, and the charging of the capacitor 5C is performed through a resistor 5R. Therefore, the fluctuation in current at the output side of the power supply circuit 2 is moderated. Accordingly, in a case where the communication device 3 performs communication in a GSM mode, influence caused by voltage fluctuation is reduced on the other circuits 4.
However, when the configuration shown in FIG. 9 is adopted, a voltage drop is produced by the resistor 5R; therefore, it is required to increase output voltage of the power supply circuit 2 accordingly. In addition, current is always consumed by the resistor 5R even in a case where communication is performed in a WCDMA mode, and this leads to an increase in power loss.